Let me take a stab at this and show you a few things you can tweak. Obviously there is no right or wrong way to do this and you should see what works best for you from the following:
A. Resolution and Frame rate.
Go to Settings > Camera > Record Video
Here I would pick a minimum of 1080p HD at 60 frames. No need to go below this, as you would see more compression artifacts/pixelation when you play your video on a TV or bigger screen.
I still think 1080p at 60 frames is better than 4K with 30 frames. So the next step up I think is 4K 60 frames that is meaningfully increasing your picture quality.
B. Disable HDR video.
Unless you are looking at your videos through a high end Dolby Vision capable TV set, I would disable this setting.
C. Turn off Auto FPS.
This can also reduce the file size without much compromise in video quality.
D. Disable Stereo
Go to Settings > Camera > Record Sound. Set it to Mono
This will reduce the file size further. You might want to try Stereo, but the Spatial option is not going to yield more noticeable improvements.
Remember that there will always be a tradeoff. Better quality video will result in larger file size. By reducing file size, you're giving up a bit more quality.
